Alkali cellulose



Patented Jan. 17, 1939 UNITED STATES PATENT OFFlCE ALKALI CELLULOSE poration of Michigan No Drawing. Application May 26, 1937, Serial No. 144,963

3 Claims.

,, 1y etherified cellulose derivatives, these derivatives are often quite unsatisfactory for many commercial applications. They frequently contain gels, indicating that while the average composition of the alkali cellulose was in the range desired, it was actually non-uniform.

It is accordingly among the objects of our invention to provide a process for the preparation of alkali cellulose wherein every fiber of the cellulosic substance is completely and uniformly impregnated with alkali, and which is capable of being etherified to form uniform cellulose ethers. Another object is to provide a process for the preparation of alkali cellulose wherein every fiber of the cellulosic substance is completely and uniformly impregnated with alkali, and from which liquid alkali is not squeezed out by the application thereto of suificient pressure to halve the thickness of a swelled sheet thereof.

In our co-pending application Serial Number 144,374, filed May 24, 1937, of which this application is a continuation in part, we have described a process for the production of alkali cellulose comprising immersing an aggregate of cellulosic fibers in a bath of liquid alkali having a concentration between about and per cent at temperatures in the range from about 55 to 144 C. and preferably between about and C. until every fiber of the cellulosic aggregate has been substantially completely wet with the liquid alkali, removing the alkali-impregnated cellulosic mass from the bath together with absorbed alkali liquor, permitting the aggregate to swell to its maximum size without at any time forceably removing alkali solution therefrom, cooling the alkali cellulose to a temperature near its hardening point, and shredding the alkali cellulose. It is the swelling step mentioned above with which the present invention is concerned.

By way of illustration the invention herein is described with reference to the handling and treatment of a continuously moving sheet of cellulosic fibers, but it is to be understood that it is applicable to the treatment of any aggregate of cellulosic fibers whether or not these be in sheet form, and that it may be applied to batch as Well as to continuous processes.

In a preferred method of carrying out our invention, a sheet of cellulosic fibers is continuously unwound from a supply roll and is led at a uniform rate into and through a bath of sodium 10 hydroxide having a concentration of above 60 per cent, preferably in the range of '75 to 78.5 per cent and maintained at a temperature above the freezing point of the solution, i. e. from 55 to 144 C. and preferably from 90 to 120 C. 15 The cellulosic sheet after having been impreg nated in the alkali bath is withdrawn therefrom, excess adhering liquid alkali flowing off from the exterior of the sheet and returning to the bath. We define excess adhering liquid alkali as the amount of alkaline liquor mechanically carried from the alkali bath on the surface of the continuously moving sheet, such externally adhering alkali being unnecessary for the production of a satisfactory alkali cellulose. In our process, when the cellulosic sheet is removed from the bath, a portion of the desired liquid alkali has penetrated the fibers and has been absorbed by them, another portion remains in the interstices of the sheet, and a third portion may be carried out of the bath on the surface of the sheet due to the speed with which said sheet passes through the bath. It is necessary, therefore, to remove a part of the excess adhering liquid alkali from the surface of the cellulosic sheet. It is essential at this point that no excessive pressure be applied as the interstices of the sheet contain a large quantity of liquid alkali that is not yet absorbed by the fibers. This liquid alkali should not be removed from the sheet but should be allowed to remain so that the subsequent swelling phenomenon caused by its absorption into the fibers may take place. The alkali impregnated sheet is passed into a conditioning chamber, the temperature of which is ordinarily at or above 40 C. to prevent the alkali in the impregnated sheet from solidifying, or if desired, the exit end of the conditioning chamber may be cooled so as to effect solidification of the alkali in the sheet after the alkali cellulose has swelled to its maximum thickness. From the conditioning chamber, the alkali cellulose sheet is drawn through a set of compression rollers which are spaced apart a distance ordinarily less than half the thickness of the swelled sheet. Under the aforesaid conditions, no liquid alkali is removed from the sheet at this point, as it has been completely absorbed by the cellulose fibers. After passing the compression rollers the alkali cellulose is shredded or otherwise disintegrated and is ready for etherification.

We have found that if our alkali cellulose is compressed immediately after leaving the alkali bath in which it has been impregnated and be fore swelling has occurred to the maximum eX- tent possible, large quantities of alkali liquor are expressed from the sheet and cellulose ethers produced from such alkali cellulose are non-uniform and are generally unsatisfactory for many applications. If, however, the alkali solution car ried out from the bath in the interstices of the cellulosic mass is permitted to remain in contact with the cellulosic fibers until each of these has swelled to its maximum diameter, i. e. until the sheet itself has swelled to its maximum thickness, no liquid alkali will be removed from the sheet by applying working pressure thereto sufiicient to halve the thickness of said sheet. Alkali cellulose prepared in this manner, when etherified under the same conditions as those employed with the alkali cellulose which was squeezed immediately after leaving the bath, forms uniform cellulose ethers which are generally acceptable to the trade.

The length of time required to effect complete swelling of the cellulosic aggregate varies considerably depending on the nature of the fibers therein and on the ability of this aggregate to absorb liquid alkali. Cellulosic aggregates of any desired size or sheets of any thickness may be employed, provided, that their period of contact with the liquid alkali is adjusted so as to permit the desired degree of impregnation to take place, and, provided further, that the temperature of the atmosphere in the conditioning chamber to which the impregnated mass is conveyed after its immersion in the alkali bath is sufficiently high so that the alkali remains in a liquid state until it has been completely absorbed by the fibers.

The term moderate compression employed in the following claims means that moderate working pressure which we have employed in our process. More particularly, it is the amount of pressurev required to reduce by about one-half the thickness of a sheet of alkali cellulose swelled to its maximum dimension, and corresponds to the pressure exerted by our compression rollers on a swelled sheet of alkali cellulose between the conditioning chamber and the disintegrating or shredding mechanism. We do not imply that liquid alkali could not be squeezed from a swelled sheet of alkali cellulose if extreme pressures were applied thereto, e. g. if subjected to the pressure of a hydraulic ram.

While we have ordinarily carried out our invention by employing a sodium hydroxide solution, thus forming soda cellulose, our process is equally applicable to the employment of potassium hydroxide or mixtures thereof with sodium hydroxide to produce potash cellulose or mixed soda-potash cellulose.

Other modes of applying the principle of our invention may be employed instead of the one explained, change being made as regards the process herein disclosed, provided the step or steps stated by any of the following claims or the equivalent of such stated step or steps be employed.

We, therefore, particularly point out and distinctly claim as our invention:

1. In a process for the preparation of uniform alkali cellulose adapted to the production of cel lulose ethers, the steps which consist in immersing an aggregate of cellulosic fibers in a bath of aqueous alkali having an alkali concentration between about 60 and about 80 per cent until the cellulosic aggregate has absorbed an amount of alkali which, after the aggregate has swelled to its maximum dimension, will remain therein when said aggregate is subjected to that amount of compression which will reduce the thickness of the aggregate by about one-half, then removing said aggregate from the bath and maintaining the temperature thereof above the freezing point of the alkali therein contained until said aggregate has swelled substantially to its maximum size.

2. In a process for the preparation of uniform alkali cellulose adapted to the production of cellulose ethers, the steps which consist in immersing a sheet of cellulosic fibersin a bath of aqueous alkali having an alkali concentration between about 60 and about 80 per cent until the cellulosic sheet has absorbed an amount of alkali which, after the sheet has swelled to its maximum thickness, will remain therein when said sheet is subjected to that amount of compression which will reduce the thickness of the sheet by about one-half, then removing said sheet from the bath and maintaining the temperature thereof above the freezing point of the alkali therein contained until said sheet has swelled substantially to its maximum thickness.

3. In a process for the preparation of uniform alkali cellulose adapted to the production of cellulose ethers, the steps which consist in immersing an aggregate of cellulosic fibers in a bath of aqueous alkali having an alkali concentration between about 75 and about 78.5 per cent until the cellulosic aggregate has absorbed an amount of alkali which, after the aggregate has swelled to its maximum dimension, will remain therein when said aggregate is subjected to that amount of compression which will reduce the thickness of the aggregate by about one-half, then removing said aggregate from the bath and maintaining the temperature thereof above the freezing point of alkali therein contained untilsaid aggregate has swelled substantially to its maximum size.

WILLIAM R. COLLINGS. LEE DE PREE. MERRILL H. WEYMOUTH. 

